1. Field of the Invention
The invention relates to a method and an arrangement a transmission method and a multi-channel transmission system.
The invention is used in a COFDM (Coded Orthogonal Frequency Division Multiplexing) transmission system that has been proposed for digital radio.
2. Background Information
COFDM is a digital multi-channel modulation method. In such a method, the data signal to be transmitted is divided to a number of N (e.g. several 100) subchannels which lie next to one another in the frequency domain, with their spectra possibly also overlapping. With this division, the data rate transmitted in each subchannel is only a fraction of the original. The symbol duration is extended in the same ratio which has an advantageous effect if echoes appear on the transmission path. By selecting N to be sufficiently high, it is always possible for the symbol duration to be long relative to the maximum echo delay. Thus the symbol interference caused by echoes is reduced to the extent that the signal can be demodulated without distortion.
The COFDM transmission signal s(t) can be represented in the base band as a superposition of time and frequency shifted basic pulses b(t): ##EQU1## The basic pulse is here given by ##EQU2## j=.sqroot.-1 is the imaginary unit.
The summation index i represents the symbol clock, index k represents the subchannel. The following parameters describe the COFDM modulation method:
T.sub.s : symbol duration PA1 t.sub.s : utilized symbol duration PA1 t.sub.g : protection period PA1 F.sub.s : subchannel spacing
They are related by way of the equations T.sub.s =t.sub.s +t.sub.g and F.sub.s =1/t.sub.s. The quotient of the utilized symbol duration and the symbol duration, .gamma.=t.sub.s /T.sub.s can be defined as a further parameter. In the COFDM variations presently being discussed for use in digital radio, it always applies that .gamma.=0.8. Under consideration of these relationships, the COFDM system has only one free parameter from which the remaining can be derived.
The information to be transmitted is coded in complex symbols d.sub.i,k. In COFDM, 4-phase keying is employed as the modulation method; it therefore applies that d.sub.i,k .epsilon.(1, j, -1, -j). In order for the transmission to be insensitive to channel specific phase shifts, it is not the d.sub.i,k symbols that are transmitted but the transmission signals s.sub.i,k produced by differential coding s.sub.i,k =s.sub.i-1,k .multidot.d.sub.i,k. Sometimes it is practical to combine the transmission symbols of all subchannels that were transmitted during the same time slot i into an N-dimensional vector s.sub.i. Such a vector is called a symbol block.
The generation of the COFDM transmission signal is effected, for example, digitally with the aid of the inverse fast Fourier transformation (IFFT). The block circuit diagram of a COFDM transmitter is shown in FIG. 14a. An IFFT is calculated for each time slot i. The output signal of the IFFT has the duration t.sub.s. It is continued periodically to become a signal of the duration T.sub.s.
The COFDM demodulator serves to recover the information carrying symbols d.sub.i,k. For this purpose, the following values are formed from the receiver input signal r(t): ##EQU3## From this value, estimated values d.sub.i,k are derived for the data symbols by differential demodulation d.sub.i,k =r.sub.i,k r*.sub.i-1,k, where r* is the conjugate complex to r.
The COFDM demodulator is also realizable digitally with the aid of the fast Fourier transformation (FFT). It is shown in FIG. 14b. A section of the duration t.sub.s of the received signal is evaluated for every time slot. One section of the duration t.sub.g remains unevaluated. The echoes of the signal from the preceding time slot fall into this section.
Data transmission in the COFDM system is frame oriented. A frame is a structured arrangement of timely successive symbol blocks. It has the following structure shown in FIG. 15:
The first symbol block s.sub.1 in the frame is the zero symbol. It is characterized by the fact that no transmission signal is propagated. By means of an envelope detector, the receiver is able to detect the break in the field intensity. The distance between the zero symbols of the n.sup.th and the (n+1).sup.th frame serves to synchronize the frames, the duration of the zero symbol serves to synchronize the symbols. The zero symbol is not processed by means of the FFT. It generally has a length other than T.sub.s.
The second symbol block s.sub.2 in the frame is the phase reference symbol. It is required to initialize the differential demodulator. It is a complex sweep signal s.sub.2,k =exp (j .pi. k.sup.2 /N).
The remainder of the frame is composed of information carrying symbol blocks.
COFDM requires that the carrier frequencies of transmitter and receiver match very precisely. The maximum tolerated deviation lies in an order of magnitude of 5% of the subchannel spacing F.sub.s. This can be realized only with very expensive special oscillators which are not suitable for mass production. It is better to employ a controlled oscillator in the receiver. At the moment of turn-on, however, this oscillator may have a frequency deviation in the order of magnitude of several subchannel spacings.